If he ___ me his telephone number yesterday, I ___ able to ring him up now.

A．told, would be

B．had told, would have been

C．had told, would be

D．told, am

北京旧城的格局形成于_________、________两代，成________形。在这之前旧称城区曾几度变迁，引起变迁的自然原因主要是___________________。